Liquid crystal display device

ABSTRACT

A first photo-alignment film ( 12 ) of a liquid crystal display device ( 100 ) includes a first and a second pre-tilt region ( 12   a,    12   b ) defining pre-tilt directions (PD 1 , PD 2 ) that are anti-parallel to each other, and a second photo-alignment film ( 22 ) thereof includes a third and a fourth pre-tilt region ( 22   a,    22   b ) defining pre-tilt directions (PD 3 , PD 4 ) that are anti-parallel to each other. The entire boundary (BD 1 ) between the first and second pre-tilt regions and the entire boundary (BD 2 ) between the third and fourth pre-tilt regions are aligned with each other, as seen from the display plane normal direction. A pixel electrode ( 11 ) includes a first and a second cut-off portion ( 11   a   1, 11   a   2 ) provided by cutting off at least a part of a particular edge portion ( 11   e   1, 11   e   2 ) of the outer perimeter thereof.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device, andparticularly to a liquid crystal display device including a liquidcrystal layer of a vertical alignment type, wherein the pre-tiltdirection of liquid crystal molecules is defined by a photo-alignmentfilm.

BACKGROUND ART

In recent years, liquid crystal display devices, whose displaycharacteristics have improved, are more and more used in televisionreceivers, or the like. Although viewing angle characteristics of liquidcrystal display devices have improved, there is a demand for furtherimprovements. Particularly, there is a strong demand for improvingviewing angle characteristics of liquid crystal display devices using aliquid crystal layer of a vertical alignment type (referred to also asliquid crystal display devices of a VA mode).

At present, liquid crystal display device of a VA mode, which are usedin large-size liquid crystal display devices such as televisions, employan alignment-divided structure in which a plurality of liquid crystaldomains are formed in one pixel in order to improve the viewing anglecharacteristic. A mainstream method for forming an alignment-dividedstructure is the MVA mode. The MVA mode is disclosed in Patent DocumentNo. 1, for example.

In the MVA mode, by providing an alignment regulating structure on theliquid crystal layer side of each of a pair of substrates that areopposing each other with a vertical alignment-type liquid crystal layerinterposed therebetween, there are formed a plurality of liquid crystaldomains of different orientation directions (tilt directions) (typicallyof four different orientation directions) within each pixel. Thealignment regulating structure may be slits (openings) provided inelectrodes or ribs (projecting structures), exerting alignmentregulating forces from both sides of the liquid crystal layer.

Using slits and ribs, however, the alignment regulating force acting onliquid crystal molecules becomes non-uniform, thereby resulting in adistribution of response speed, because slits or ribs are linear, asopposed to cases where the pre-tilt direction is defined by an alignmentfilm as in the conventional TN mode. Moreover, the display brightnesslowers because the optical transmittance lowers in areas where slits orribs are provided.

In order to avoid such a problem, it is preferred to form analignment-divided structure by defining the pre-tilt direction using analignment film, also with liquid crystal display devices of a VA mode. Aliquid crystal display device of the VA mode, in which analignment-divided structure is formed as described above, has beenproposed in Patent Document No. 2 by the present applicant.

In the liquid crystal display device disclosed in Patent Document No. 2,the pre-tilt direction is defined by an alignment film, thereby forminga 4-divided alignment structure. That is, in the presence of a voltageapplied across the liquid crystal layer, there are formed four liquidcrystal domains within one pixel. Such a 4-divided alignment structuremay also be referred to simply as a 4D structure.

In the liquid crystal display device disclosed in Patent Document No. 2,the pre-tilt direction defined by one of a pair of alignment films thatare opposing each other with a liquid crystal layer interposedtherebetween is generally 90° apart from that of the other alignmentfilm. Therefore, the liquid crystal molecules assume a twistedorientation in the presence of an applied voltage. The VA mode, in whichthe liquid crystal molecules assume a twisted orientation due to the useof a pair of vertical alignment films provided so that their pre-tiltdirections (alignment treatment directions) are orthogonal to each otheras described above, may be referred to also as the VATN (VerticalAlignment Twisted Nematic) mode or the RTN (Reverse Twisted Nematic)mode. As already described above, since a 4D structure is formed by theliquid crystal display device of Patent Document No. 2, the presentapplicant refers to the display mode of the liquid crystal displaydevice of Patent Document No. 2 as the 4D-RTN mode.

As a specific method for allowing the pre-tilt direction of the liquidcrystal molecules to be defined by an alignment film, methods in which aphoto-alignment treatment is performed as described in Patent DocumentNo. 2 have been considered promising. Since the photo-alignmenttreatment can be done with no direct contact, no static electricity willoccur due to friction as in a rubbing treatment, and it is possible toimprove the production yield. Patent Document No. 3 also discloses aliquid crystal display device of the VAIN mode using an alignment film(photo-alignment film) having been subjected to a photo-alignmenttreatment.

CITATION LIST Patent Literature

[Patent Document No. 1] Japanese Laid-Open Patent Publication No.11-242225

[Patent Document No. 2] International Publication WO2006/132369

[Patent Document No. 3] International Publication WO2006/121220

SUMMARY OF INVENTION Technical Problem

In recent years, however, the definition of the liquid crystal displaydevice has increased, and a study by the present inventors has indicatedthat a display defect may occur (particularly, while playing a movie),if a VAIN mode using a photo-alignment film is employed for ahigh-definition liquid crystal display device. Specifically, it has beenfound that with high-definition pixel designs for medium- to small-sizeapplications, the stability of the orientation of liquid crystalmolecules or the response speed thereof may be insufficient.

The present invention, which has been made in view of the problems setforth above, has an object to provide a liquid crystal display device ofthe VA mode which is suitable for higher definitions and in which thepre-tilt direction of the liquid crystal molecules is defined by aphoto-alignment film.

Solution to Problem

A liquid crystal display device according to an embodiment of thepresent invention includes a plurality of pixels arranged in a matrixpattern, the liquid crystal display device including: a first substrateand a second substrate arranged so as to oppose each other; and a liquidcrystal layer of a vertical alignment type provided between the firstsubstrate and the second substrate, wherein: the first substrateincludes a pixel electrode provided in each of the plurality of pixels,and a first photo-alignment film provided between the pixel electrodeand the liquid crystal layer; the second substrate includes a counterelectrode opposing the pixel electrode, and a second photo-alignmentfilm provided between the counter electrode and the liquid crystallayer; the first photo-alignment film has, in each of the plurality ofpixels, a first pre-tilt region defining a first pre-tilt direction, anda second pre-tilt region defining a second pre-tilt direction, which isanti-parallel to the first pre-tilt direction; the secondphoto-alignment film has, in each of the plurality of pixels, a thirdpre-tilt region defining a third pre-tilt direction, and a fourthpre-tilt region defining a fourth pre-tilt direction, which isanti-parallel to the third pre-tilt direction; as seen from a displayplane normal direction, an entire boundary between the first pre-tiltregion and the second pre-tilt region of the first photo-alignment filmand an entire boundary between the third pre-tilt region and the fourthpre-tilt region of the second photo-alignment film are aligned with eachother; an outer perimeter of the pixel electrode includes a first edgeportion and a second edge portion; a direction which is orthogonal tothe first edge portion and which extends toward inside of the pixelelectrode is opposite to the first pre-tilt direction; a direction whichis orthogonal to the second edge portion and which extends toward insidethe pixel electrode is opposite to the second pre-tilt direction; andthe pixel electrode includes a first cut-off portion provided by cuttingoff at least a part of the first edge portion, and a second cut-offportion provided by cutting off at least a part of the second edgeportion.

In one embodiment, the first cut-off portion has a right triangle shapeobtained by cutting off a corner of the pixel electrode in the vicinityof the first edge portion; and the second cut-off portion has a righttriangle shape obtained by cutting off a corner of the pixel electrodein the vicinity of the second edge portion.

In one embodiment, where a₁, a₂ and b denote lengths of the firstcut-off portion, the second cut-off portion and the pixel electrode,respectively, along a direction orthogonal to the first pre-tiltdirection and the second pre-tilt direction, the length a₁ of the firstcut-off portion, the length a₂ of the second cut-off portion and thelength b of the pixel electrode satisfy relationships a₁/b≧0.25 anda₂/b≧0.25.

In one embodiment, the length a₁ of the first cut-off portion, thelength a₂ of the second cut-off portion and the length b of the pixelelectrode satisfy relationships a₁/b≦0.5 and a₂/b≦0.5.

In one embodiment, an angle φ₁ formed between a hypotenuse of the firstcut-off portion and the first edge portion and an angle φ₂ formedbetween a hypotenuse of the second cut-off portion and the second edgeportion satisfy relationships φ₁≧1° and φ₂≧1°.

In one embodiment, as seen from the display plane normal direction, thefirst pre-tilt region of the first photo-alignment film and the thirdpre-tilt region of the second photo-alignment film are aligned with eachother and the second pre-tilt region of the first photo-alignment filmand the fourth pre-tilt region of the second photo-alignment film arealigned with each other; and the third pre-tilt direction isanti-parallel to the first pre-tilt direction, and the fourth pre-tiltdirection is anti-parallel to the second pre-tilt direction.

In one embodiment, when a voltage is applied between the pixel electrodeand the counter electrode, four liquid crystal domains are formed in theliquid crystal layer in each of the plurality of pixels; and azimuthdirections of four directors representing orientation directions ofliquid crystal molecules included in the four liquid crystal domains,respectively, are different from each other.

In one embodiment, the liquid crystal display device having such aconfiguration as described above further includes a pair of linearpolarizers which are arranged so as to oppose each other with the liquidcrystal layer interposed therebetween and so that transmission axesthereof are generally orthogonal to each other, wherein the transmissionaxes of the pair of linear polarizers form an angle of generally 45°with respect to the first pre-tilt direction.

In one embodiment, the liquid crystal display device having such aconfiguration as described above further includes a pair of circularpolarizers opposing each other with the liquid crystal layer interposedtherebetween.

In one embodiment, the liquid crystal layer includes liquid crystalmolecules having a negative dielectric anisotropy.

In one embodiment, a shorter one of a pixel pitch along a display planehorizontal direction and a pixel pitch along a display plane verticaldirection is 42 μm or less.

In one embodiment, a screen resolution is 200 ppi or more.

Advantageous Effects of Invention

According to an embodiment of the present invention, there is provided aliquid crystal display device of a VA mode which is suitable forhigher-definition applications, and in which the pre-tilt direction ofliquid crystal molecules is defined by a photo-alignment film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A cross-sectional view schematically showing a liquid crystaldisplay device 100 according to an embodiment of the present invention.

FIG. 2 A diagram showing an alignment-divided structure of a pixel 1001in a liquid crystal display device 1000 of a common 4D-RTN mode.

FIGS. 3 (a), (b) and (c) are diagrams illustrating a method forobtaining the alignment-divided structure of the pixel 1001 shown inFIG. 2.

FIG. 4 A diagram showing an alignment-divided structure of a pixel 1 inthe liquid crystal display device 100.

FIGS. 5 (a), (b) and (c) are diagrams illustrating a method forobtaining the alignment-divided structure of the pixel 1 shown in FIG.4.

FIG. 6 A diagram schematically showing liquid crystal domains to beformed when taking into consideration only the alignment regulatingforces of a first photo-alignment film 12 and a second photo-alignmentfilm 22.

FIGS. 7 (a) and (b) are diagrams schematically showing a pixel 901 of aliquid crystal display device 900 of a reference example.

FIG. 8 (a) to (d) are views illustrating the results of an orientationsimulation for verifying the orientation of the liquid crystal moleculesin the presence of an applied voltage, for a liquid crystal displaydevice 1000 of a 4D-RTN mode.

FIG. 9 (a) to (d) are views illustrating the results of an orientationsimulation for verifying the orientation of the liquid crystal moleculesin the presence of an applied voltage for a liquid crystal displaydevice 900 of a reference example.

FIG. 10 A diagram illustrating the reason why four liquid crystaldomains are formed in a pixel 901 of the liquid crystal display device900 of a reference example.

FIGS. 11 (a) and (b) are diagrams illustrating the reason why fourliquid crystal domains are formed in a pixel 901 of the liquid crystaldisplay device 900 of a reference example.

FIGS. 12 (a) and (b) are diagrams illustrating the reason why fourliquid crystal domains are formed in a pixel 901 of the liquid crystaldisplay device 900 of a reference example.

FIGS. 13 (a) and (b) are diagrams illustrating the reason why fourliquid crystal domains are formed in a pixel 901 of the liquid crystaldisplay device 900 of a reference example.

FIG. 14 A diagram illustrating the reason why a disclination occurs in apixel 901 of the liquid crystal display device 900 of a referenceexample.

FIG. 15 A plan view schematically showing a pixel 1 of the liquidcrystal display device 100.

FIG. 16 A diagram illustrating the reason why the occurrence of adisclination is suppressed in a pixel 1 of the liquid crystal displaydevice 100.

FIGS. 17 (a) and (b) are diagrams illustrating the reason why theoccurrence of a disclination is suppressed in a pixel 1 of the liquidcrystal display device 100.

FIG. 18 A plan view schematically showing a pixel 1 of the liquidcrystal display device 100.

FIG. 19 Views illustrating the results of an orientation simulation whenthe cut-off angle is 0° and the cut-off ratio is 0.

FIG. 20 Views illustrating the results of an orientation simulation whenthe cut-off angle is 1° and the cut-off ratio is 0.25.

FIG. 21 Views illustrating the results of an orientation simulation whenthe cut-off angle is 1° and the cut-off ratio is 0.50.

FIG. 22 Views illustrating the results of an orientation simulation whenthe cut-off angle is 2° and the cut-off ratio is 0.10.

FIG. 23 Views illustrating the results of an orientation simulation whenthe cut-off angle is 2° and the cut-off ratio is 0.25.

FIG. 24 Views illustrating the results of an orientation simulation whenthe cut-off angle is 2° and the cut-off ratio is 0.50.

FIG. 25 Views illustrating the results of an orientation simulation whenthe cut-off angle is 5° and the cut-off ratio is 0.10.

FIG. 26 Views illustrating the results of an orientation simulation whenthe cut-off angle is 5° and the cut-off ratio is 0.25.

FIG. 27 Views illustrating the results of an orientation simulation whenthe cut-off angle is 15° and the cut-off ratio is 0.05.

FIG. 28 Views illustrating the results of an orientation simulation whenthe cut-off angle is 15° and the cut-off ratio is 0.10.

FIG. 29 Views illustrating the results of an orientation simulation whenthe cut-off angle is 30° and the cut-off ratio is 0.05.

FIG. 30 Views illustrating the results of an orientation simulation whenthe cut-off angle is 30° and the cut-off ratio is 0.10.

FIG. 31 Views illustrating the results of an orientation simulation whenthe cut-off angle is 45° and the cut-off ratio is 0.05.

FIG. 32 Views illustrating the results of an orientation simulation whenthe cut-off angle is 45° and the cut-off ratio is 0.10.

FIG. 33 A plan view schematically showing a pixel 1 of a liquid crystaldisplay device 100 when the cut-off ratio is 0.25.

FIG. 34 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 1.

FIG. 35 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 2.

FIG. 36 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 3.

FIG. 37 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 4.

FIG. 38 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 5.

FIG. 39 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Reference Example 6.

FIG. 40 (a) to (e) are views illustrating the results of an orientationsimulation and an optical simulation of Comparative Example 1.

FIGS. 41 (a), (b) and (c) are views, for Comparative Example 1,Reference Example 6 and Reference Example 2, respectively, each showingthe orientation distribution within a pixel.

FIGS. 42 (a) and (b) are graphs, for Reference Example 7 and ComparativeExample 2, respectively, each showing the relationship between the graylevel and the brightness (normalized brightness) when observed from thefront direction, that when observed from a diagonally left/right 60°direction, and that when observed from a diagonally upper/lower 60°direction.

FIGS. 43 (a) and (b) are a cross-sectional view and a plan view,respectively, schematically showing one pixel of a liquid crystaldisplay device 800 of a CPA mode, and (c) is a view showing thesimulation results for the transmittance when a white voltage is appliedacross a liquid crystal layer 830 of the liquid crystal display device800.

FIGS. 44 (a), (b) and (c) are diagrams showing the results of verifyingthe formation of an undesirable tilt region NGR due to the opticaldiffraction phenomenon.

FIG. 45 A diagram illustrating the results of verifying the formation ofthe undesirable tilt region NGR due to the optical diffractionphenomenon.

FIG. 46 A graph showing the position profile along the direction Y ofthe pre-tilt angle (the left-right direction of FIG. 45) for theenhanced region, the intermediate region and the offset region.

FIGS. 47 (a) and (b) are graphs, for two liquid crystal display devices900 prototyped as Example 7, each showing the distribution of thepre-tilt angle within a pixel 901.

FIG. 48 An optical microscope image of one pixel 901 of ReferenceExample 7.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings. Note that the present invention is not limited to thefollowing embodiments.

FIG. 1 shows a liquid crystal display device 100 of the presentembodiment. FIG. 1 is a cross-sectional view schematically showing theliquid crystal display device 100.

As shown in FIG. 1, the liquid crystal display device 100 includes anactive matrix substrate (first substrate) 10 and a counter substrate(second substrate) 20 arranged so as to oppose each other, and a liquidcrystal layer 30 provided between the active matrix substrate 10 and thecounter substrate 20. The liquid crystal display device 100 furtherincludes a plurality of pixels arranged in a matrix pattern. Typically,the plurality of pixels include red pixels for displaying red, greenpixels for displaying green, and blue pixels for displaying blue, andthree pixels (a red pixel, a green pixel and a blue pixel) together formone color display pixel.

An active matrix substrate (referred to also as a “TFT substrate”) 10includes a pixel electrode 11 provided in each of the plurality ofpixels, and a first photo-alignment film 12 provided between a pixelelectrode 11 and the liquid crystal layer 30. The pixel electrode 11 andthe first photo-alignment film 12 are supported on an insulativetransparent substrate (e.g., a glass substrate) 10 a. Although not shownin the figures, the active matrix substrate includes a thin filmtransistor (TFT) electrically connected to the pixel electrode 11, ascanning line (gate bus line) for supplying a scanning signal to a TFT,a signal line (source bus line) for supplying a display signal to a TFT,etc.

The counter substrate (referred to also as a “color filter substrate”)20 includes a counter electrode 21 opposing the pixel electrode 11, andthe second photo-alignment film 22 provided between the counterelectrode 21 and the liquid crystal layer 30. The counter electrode 21and the second photo-alignment film 22 are supported on an insulativetransparent substrate (e.g., a glass substrate) 20 a. Although not shownin the figures, the counter substrate 20 includes a color filter layer.Typically, the color filter layer includes red color filters provided soas to correspond to red pixels, green color filters provided so as tocorrespond to green pixels, and blue color filters provided so as tocorrespond to blue pixels.

The liquid crystal layer 30 is a liquid crystal layer of a verticalalignment type, including liquid crystal molecules 31 having a negativedielectric anisotropy. In the absence of a voltage applied across theliquid crystal layer 30, the liquid crystal molecules 31 are orientedgenerally vertical to the substrate surface, as shown in FIG. 1.

The liquid crystal display device 100 further includes a pair of linearpolarizers 18 and 28 opposing each other with at least a liquid crystallayer 30 interposed therebetween. The linear polarizers 18 and 28 arearranged so that their transmission axes are generally orthogonal toeach other. That is, the linear polarizers 18 and 28 are arranged in across-Nicol arrangement. Note that a pair of circular polarizers may beprovided instead of the pair of linear polarizers 18 and 28. That is,light to be incident on the liquid crystal layer 30 may be eitherlinearly-polarized light or circularly-polarized light.

The first photo-alignment film 12 and the second photo-alignment film 22are each a vertical alignment film having been subjected to aphoto-alignment treatment, and define the pre-tilt direction of theliquid crystal molecules 31. The first photo-alignment film 12 has tworegions, within each pixel, defining different pre-tilt directions fromeach other. Similarly, the second photo-alignment film 22 has tworegions, within each pixel, defining different pre-tilt directions fromeach other.

The alignment-divided structure formed by the first photo-alignment film12 and the second photo-alignment film 22 of the liquid crystal displaydevice 100 of the present embodiment will be described below, followingthe description of the alignment-divided structure of the 4D-RTN mode asdisclosed in Patent Document No. 2 and Patent Document No. 3.

FIG. 2 shows an alignment-divided structure of a pixel 1001 in a liquidcrystal display device 1000 of a common 4D-RTN mode. In the presence ofa voltage applied across the liquid crystal layer, four liquid crystaldomains A, B, C and D are formed in the pixel 1001 as shown in FIG. 2.The four liquid crystal domains A, B, C and D are arranged in a 2-by-2matrix pattern. Note that FIG. 2 also shows a pixel electrode 1011provided in the pixel 1001.

The azimuth directions of the directors t1, t2, t3 and t4 of the liquidcrystal domains A, B, C and D are four azimuth directions, of which thedifference between any two directions is generally equal to an integermultiple of 90°. Each of the directors t1, t2, t3 and t4 is arepresentation of the orientation direction of the liquid crystalmolecules included in the liquid crystal domain, and it in the 4D-RTNmode is the tilt direction of liquid crystal molecules in the vicinityof the center on the layer plane and in the thickness direction of theliquid crystal layer in the presence of a voltage applied across theliquid crystal layer. Each liquid crystal domain is characterized by theazimuth direction of the director (the tilt direction described above),and the azimuth direction of the director has a dominant influence onthe viewing angle dependence of the domain.

Note that a pair of polarizers opposing each other with the liquidcrystal layer interposed therebetween are arranged so that thetransmission axes (polarization axes) are orthogonal to each other, andmore specifically, they are arranged so that one transmission axis isparallel to the horizontal direction of the display plane, and the othertransmission axis is parallel to the vertical direction to the displayplane.

Assuming that the azimuth angle (the 3 o'clock direction) in thehorizontal direction on the display plane is 0°, the azimuth directionof the director t1 of the liquid crystal domain A is a generally 225°direction, the azimuth direction of the director t2 of the liquidcrystal domain B is a generally 315° direction, the azimuth direction ofthe director t3 of the liquid crystal domain C is a generally 45°direction, and the azimuth direction of the director t4 of the liquidcrystal domain D is a generally 135° direction. That is, the liquidcrystal domains A, B, C and D are arranged so that the azimuthdirections of the directors are generally 90° apart from each otherbetween adjacent liquid crystal domains.

Now, referring to FIGS. 3(a), (b) and (c), an alignment-dividing methodfor obtaining the alignment-divided structure of the pixel 1001 shown inFIG. 2 will be described. FIG. 3(a) shows pre-tilt directions PD1 andPD2 defined by the photo-alignment film provided on the active matrixsubstrate, and FIG. 3(b) shows pre-tilt directions PD3 and PD4 definedby the photo-alignment film provided on the counter substrate. FIG. 3(c)shows tilt directions (directors) in the presence of a voltage appliedacross the liquid crystal layer after the active matrix substrate andthe counter substrate are attached together.

A region on the active matrix substrate side (a region corresponding toone pixel 1001) is divided into two regions in the left-right direction,as shown in FIG. 3(a), and the photo-alignment treatment is performed sothat the photo-alignment films (vertical alignment films) of theseregions (the left region and the right region) define the pre-tiltdirections PD1 and PD2 that are anti-parallel to each other. Here, thephoto-alignment treatment is performed through diagonal irradiations ofultraviolet light from directions indicated by the arrows.

On the other hand, a region on the counter substrate side (a regioncorresponding to one pixel region 1001) is divided into two regions inthe up-down direction, as shown in FIG. 3(b), and the photo-alignmenttreatment is performed so that the photo-alignment films (verticalalignment films) of these regions (the upper region and the lowerregion) define the pre-tilt directions PD3 and PD4 that areanti-parallel to each other. Here, the photo-alignment treatment isperformed through diagonal irradiations of ultraviolet light fromdirections indicated by the arrows.

A pixel 1001 that is alignment-divided as shown in FIG. 3(c) can beformed by attaching together an active matrix substrate and a countersubstrate which have been subjected to a photo-alignment treatment asshown in FIGS. 3(a) and (b). As can be seen from FIGS. 3(a), (b) and(c), for each of the liquid crystal domains A to D, the pre-tiltdirection defined by the photo-alignment film of the active matrixsubstrate and the pre-tilt direction defined by the photo-alignment filmof the counter substrate are generally 90° apart from each other, sothat a tilt direction (the azimuth direction of the director of theliquid crystal domain) is defined along an intermediate directionbetween these two pre-tilt directions.

As already described above, a display defect may occur (particularly,while playing a movie), if such an alignment-divided structure as thatof the pixel 1001 (i.e., of a common 4D-RTN mode) is employed in ahigh-definition liquid crystal display device. The reason for this willnow be described.

When making an alignment-divided structure of the 4D-RTN mode, it isnecessary to perform the exposure step twice for each of thephoto-alignment film on the active matrix side and the photo-alignmentfilm on the counter substrate side. The ultraviolet light irradiation isperformed from different directions in the first exposure step and inthe second exposure step. Where the exposure step is performed through ascanning exposure using a photomask, there will be a region where asufficient pre-tilt angle cannot be realized (hereinafter referred to asan “undesirable tilt region”), due to the optical diffractionphenomenon, in the vicinity of an exposure boundary (the boundarybetween two regions where different pre-tilt directions are to bedefined). Note that the “pre-tilt angle”, as used in the presentspecification, refers to the angle of the long axis of the liquidcrystal molecules 31 with respect to the substrate surface normaldirection in the absence of an applied voltage.

Specifically, the photo-alignment film on the active matrix substrateside will have an undesirable tilt region NGR1 in the vicinity of theboundary BD1 between the left region and the right region, as shown inFIG. 3(a). The photo-alignment film on the counter substrate side willhave an undesirable tilt region NGR2 in the vicinity of the boundary BD2between the upper region and the lower region, as shown in FIG. 3(b).Therefore, with the active matrix substrate and the counter substrateattached together, each pixel 1001 has a cross-shaped undesirable tiltregion NGR, including a portion extending in the vertical direction(corresponding to the undesirable tilt region NGR1) and another portionextending in the horizontal direction (corresponding to the undesirabletilt region NGR2).

With high-definition pixel designs for medium- to small-sizeapplications, the undesirable pre-tilt region accounts for a largeproportion of the entire pixel 1001 since the pixel pitch is small.Therefore, the average pre-tilt angle of the entire pixel 1001 will besmall, and the orientation of the liquid crystal molecules may becomeunstable and the response speed may be low (i.e., the response time maybe long).

Then, the alignment-divided structure of a pixel in the liquid crystaldisplay device 100 of the present embodiment will be described. FIG. 4shows the alignment-divided structure of a pixel 1 in the liquid crystaldisplay device 100.

When a voltage is applied between the pixel electrode 11 and the counterelectrode 21, there are formed four liquid crystal domains A, B, C andD, in each pixel 1, in the liquid crystal layer 30, as shown in FIG. 4.The azimuth directions of the four directors t1, t2, t3 and t4representing the orientation direction of the liquid crystal molecules31 included in the four liquid crystal domains A, B, C and D,respectively, are different from each other.

Assuming that the azimuth angle (the 3 o'clock direction) in thehorizontal direction on the display plane is 0°, the azimuth directionof the director t1 of the liquid crystal domain A is a generally 270°direction, the azimuth direction of the director t2 of the liquidcrystal domain B is a generally 0° direction, the azimuth direction ofthe director t3 of the liquid crystal domain C is a generally 90°direction, and the azimuth direction of the director t4 of the liquidcrystal domain D is a generally 180° direction. That is, the differencebetween any two of the azimuth directions of the four directors of theliquid crystal domains A, B, C and D is generally equal to an integermultiple of 90°.

The transmission axes (polarization axes) P1 and P2 of the pair oflinear polarizers 18 and 28 each form an angle of generally 45° withrespect to the azimuth directions of the directors t1, t2, t3 and t4 ofthe liquid crystal domains A, B, C and D. As will be understood from thedescription below, transmission axes P1 and P2 of the pair of linearpolarizers 18 and 28 each also form an angle of generally 45° withrespect to the pre-tilt direction defined by the first photo-alignmentfilm 12 and the second photo-alignment film 22.

Note that while FIG. 4 shows an example where the four liquid crystaldomains A, B, C and D each account for an equal area within the pixel 1,the areas of the four liquid crystal domains A, B, C and D may not beequal to each other. Note however that in view of the uniformity of theviewing angle characteristic, it is preferred that the area differencebetween the four liquid crystal domains A, B, C and D is as small aspossible, and specifically, it is preferred that the difference betweenthe area of the largest one of the four liquid crystal domains A, B, Cand D and the area of the smallest liquid crystal domain is 50% or lessof the largest area. FIG. 4 shows an example of the most preferred(i.e., ideal) 4-divided structure in view of the viewing anglecharacteristic.

As shown in FIG. 4, the pixel electrode 11 of the liquid crystal displaydevice 100 of the present embodiment includes a plurality of cut-offportions (a first cut-off portion and a second cut-off portion) 11 a 1and 11 a 2.

Referring now to FIGS. 5(a), (b) and (c), an alignment-dividing methodfor obtaining an alignment-divided structure of the pixel 1 in theliquid crystal display device 100 of the present embodiment will bedescribed. FIG. 5(a) shows pre-tilt directions PD1 and PD2 defined bythe first photo-alignment film 12 provided on the active matrixsubstrate 10, and FIG. 5(b) shows pre-tilt directions PD3 and PD4defined by the second photo-alignment film 22 provided on the countersubstrate 20. FIG. 5(c) shows tilt directions (directors) in thepresence of a voltage applied across the liquid crystal layer 30 afterthe active matrix substrate 10 and the counter substrate 20 are attachedtogether.

As shown in FIG. 5(a), the first photo-alignment film 12 includes,within each pixel 1, a first pre-tilt region 12 a defining the firstpre-tilt direction PD1, and a second pre-tilt region 12 b defining thesecond pre-tilt direction PD2, which is anti-parallel to the firstpre-tilt direction PD1. Specifically, a region of the firstphoto-alignment film 12 corresponding to one pixel 1 is divided into tworegions in the up-down direction, and the photo-alignment treatment isperformed so that these regions (the first pre-tilt region and thesecond pre-tilt region) 12 a and 12 b define anti-parallel pre-tiltdirections (the first pre-tilt direction and the second pre-tiltdirection) PD1 and PD2. Here, the photo-alignment treatment is performedthrough diagonal irradiations of ultraviolet light from directionsindicated by the arrows.

On the other hand, the second photo-alignment film 22 includes, withineach pixel 1, a third pre-tilt region 22 a defining the third pre-tiltdirection PD3 and a fourth pre-tilt region 22 b defining the fourthpre-tilt direction PD4, which is anti-parallel to the third pre-tiltdirection PD3, as shown in FIG. 5(b). Specifically, a region of thesecond photo-alignment film 22 corresponding to one pixel 1 is dividedinto two regions in the up-down direction, and the photo-alignmenttreatment is performed so that these regions (the second pre-tilt regionand the third pre-tilt region) 22 a and 22 b define anti-parallelpre-tilt directions (the third pre-tilt direction and the fourthpre-tilt direction) PD3 and PD4. Here, the photo-alignment treatment isperformed through diagonal irradiations of ultraviolet light fromdirections indicated by the arrows. Note that with the active matrixsubstrate 10 and the counter substrate 20 attached together, the thirdpre-tilt region 22 a of the second photo-alignment film 22 is alignedwith (opposes) the first pre-tilt region 12 a of the firstphoto-alignment film 12, and the fourth pre-tilt region 22 b of thesecond photo-alignment film 22 is aligned with (opposes) the secondpre-tilt region 12 b of the first photo-alignment film 12, as seen fromthe display plane normal direction. With the active matrix substrate 10and the counter substrate 20 attached together, the third pre-tiltdirection PD3 is anti-parallel to the first pre-tilt direction PD1, andthe fourth pre-tilt direction PD4 is anti-parallel to the secondpre-tilt direction PD2.

A pixel 1 that is alignment-divided as shown in FIG. 5(c) can be formedby attaching together the active matrix substrate 10 and the countersubstrate 20 which have been subjected to a photo-alignment treatment asshown in FIGS. 5(a) and (b). Note that when taking into considerationonly the alignment regulating force of the first photo-alignment film 12and the second photo-alignment film 22 having been subjected to aphoto-alignment treatment as shown in FIGS. 5(a) and (b), one may thinkthat only two liquid crystal domains will be formed as in a pixel 1′shown in FIG. 6 in the presence of an applied voltage. However, forreasons to be described below, there are actually formed the four liquidcrystal domains A, B, C and D, as shown in FIG. 5(c).

There may be an undesirable tilt region NGR1, where a sufficientpre-tilt angle cannot be realized, in the vicinity of the boundary BD1between the first pre-tilt region 12 a and the second pre-tilt region 12b of the first photo-alignment film 12 (see FIG. 5(a)). There may be anundesirable tilt region NGR2, where a sufficient pre-tilt angle cannotbe realized, in the vicinity of the boundary BD2 between the thirdpre-tilt region 22 a and the fourth pre-tilt region 22 b of the secondphoto-alignment film 22. However, as shown in FIG. 5(c), in the liquidcrystal display device 100 of the present embodiment, the entireboundary BD1 between the first pre-tilt region 12 a and the secondpre-tilt region 12 b of the first photo-alignment film 12 and the entireboundary BD2 between the third pre-tilt region 22 a and the fourthpre-tilt region 22 b of the second photo-alignment film 22 are alignedwith each other, as seen from the display plane normal direction. Thus,the undesirable tilt region NGR1 on the side of the firstphoto-alignment film 12 and the undesirable tilt region NGR2 on the sideof the second photo-alignment film 22 are aligned with each other,thereby forming an undesirable tilt NGR extending only in the horizontaldirection for the entire pixel 1, as shown in FIG. 5(c).

Therefore, with the pixel 1 of the liquid crystal display device 100 ofthe present embodiment, the area of the undesirable tilt region NGR canbe made smaller than that with the pixel 1001 shown in FIG. 3(c).Therefore, since the lowering of the average pre-tilt angle of theentire pixel 1 (due to the undesirable tilt region NGR) can besuppressed, it is possible to sufficiently stabilize the orientation ofthe liquid crystal molecules 31 and to realize a sufficient responsespeed.

Now, a method for manufacturing the liquid crystal display device 100 ofthe present embodiment will be described.

First, the active matrix substrate 10 having the first photo-alignmentfilm 12 is prepared. This step can be carried out in a similar manner tothat for manufacturing an active matrix substrate of a common 4D-RTNmode. Note however that when the pixel electrode 11 is formed, aconductive film is patterned so that the first cut-off portion 11 a 1and the second cut-off portion 11 a 2 are formed.

Next, a photo-alignment treatment is performed to form the firstpre-tilt region 12 a defining the first pre-tilt direction PD1 and thesecond pre-tilt region 12 b defining the second pre-tilt direction PD2,which is anti-parallel to the first pre-tilt direction PD1, within eachof regions of the first photo-alignment film 12 corresponding to aplurality of pixels 1. This step includes, for example, a step ofirradiating with light a portion of the first photo-alignment film 12 tobe the first pre-tilt region 12 a while a portion to be the secondpre-tilt region 12 b is shaded with a photomask, and then irradiatingwith light the portion of the first photo-alignment film 12 to be thesecond pre-tilt region 12 b while the first pre-tilt region 12 a of thefirst photo-alignment film 12 is shaded with a photomask. Note that itis understood that a portion to be the second pre-tilt region 12 b maybe irradiated with light before a portion to be the first pre-tiltregion 12 a is irradiated with light.

On the other hand, the counter substrate 20 having the secondphoto-alignment film 22 is prepared, separately from the active matrixsubstrate 10. This step can be carried out in a similar manner to thatfor producing a counter substrate of a common 4D-RTN mode.

Next, a photo-alignment treatment is performed to form the thirdpre-tilt region 22 a defining the third pre-tilt direction PD3 and thefourth pre-tilt region 22 b defining the fourth pre-tilt direction PD4,which is anti-parallel to the third pre-tilt direction PD3, within eachof regions of the second photo-alignment film 22 corresponding to aplurality of pixels 1. This step includes, for example, a step ofirradiating with light a portion of the second photo-alignment film 22to be the third pre-tilt region 22 a while a portion to be the fourthpre-tilt region 22 b is shaded with a photomask, and then irradiatingwith light the portion of the second photo-alignment film 22 to be thefourth pre-tilt region 22 b while the third pre-tilt region 22 a of thesecond photo-alignment film 22 is shaded with a photomask. Note that itis understood that a portion to be the fourth pre-tilt region 22 b maybe irradiated with light before a portion to be the third pre-tiltregion 22 a is irradiated with light.

Then, the active matrix substrate 10, with the first pre-tilt region 12a and the second pre-tilt region 12 b formed on the firstphoto-alignment film 12, and the counter substrate 20, with the thirdpre-tilt region 22 a and the fourth pre-tilt region 22 b formed on thesecond photo-alignment film 22, are attached together.

Then, a vacuum injection method is used, for example, to inject a liquidcrystal material into a gap between the active matrix substrate 10 andthe counter substrate 20, thereby forming the liquid crystal layer 30.Note that it is understood that the liquid crystal layer 30 may beformed by a dripping method (i.e., applying a liquid crystal material onone of the substrates before the substrates are attached together).

The step of performing a photo-alignment treatment on the firstphoto-alignment film 12 and the step of performing a photo-alignmenttreatment on the second photo-alignment film 22 are carried out so thatwhen the active matrix substrate 10 and the counter substrate 20 areattached together, the entire boundary BD1 between the first pre-tiltregion 12 a and the second pre-tilt region 12 b of the firstphoto-alignment film 12 and the entire boundary BD2 between the thirdpre-tilt region 22 a and the fourth pre-tilt region 22 b of the secondphoto-alignment film 22 are aligned with each other, as seen from thedisplay plane normal direction.

Note that a step of performing a re-alignment treatment, including aheating treatment, on the liquid crystal layer 30 may be performed afterthe step of attaching together the active matrix substrate 10 and thecounter substrate 20. By this re-alignment treatment, it is possible toeliminate the orientation disturbance (fluid-flow orientation) occurringwhen injecting a liquid crystal material.

Then, the step of attaching the pair of linear polarizers 18 and 28 onthe outer side of the active matrix substrate 10 and the countersubstrate 20, and other steps, are performed, thereby obtaining theliquid crystal display device 100 of the present embodiment.

Note that as already described above, with the liquid crystal displaydevice 100 of the present embodiment, even though there should be formedonly two liquid crystal domains in the presence of an applied voltage,when taking into consideration only the alignment regulating force ofthe first photo-alignment film 12 and the second photo-alignment film22, there are actually formed four liquid crystal domains A, B, C and D,thereby realizing a sufficiently high viewing angle characteristic.Particularly, if the screen resolution is 200 ppi or more, it ispossible to obtain substantially the same viewing angle characteristicas that for the common 4D-RTN mode, as will be described later indetail.

With the provision of the cut-off portions (the first cut-off portion 11a 1 and the second cut-off portion 11 a 2 shown in FIG. 4, etc.) inparticular regions of the pixel electrode 11 in the liquid crystaldisplay device 100 of the present embodiment, it is possible to furtherimprove the display quality as compared with a case where such cut-offportions in the pixel electrode 11 provided in a pixel 901 are absent asin a liquid crystal display device 900 of a reference example shown inFIG. 7(a). The liquid crystal display device 900 of the referenceexample shown in FIG. 7(a) has the same configuration as the liquidcrystal display device 100 of the present embodiment except that cut-offportions are absent in the pixel electrode 11. That is, also with theliquid crystal display device 900 of the reference example, aphoto-alignment treatment is performed as described above with referenceto FIG. 5(a) to (c). Also with the liquid crystal display device 900 ofthe reference example, there are formed four liquid crystal domains A,B, C and D, thereby realizing a high viewing angle characteristic.

However, an in-depth study by the prevent inventors revealed that withthe liquid crystal display device 900 of the reference example, adisclination (orientation defect) occurs in particular regions (regionsR1 and R2 in the figure) near the outer perimeter of the pixel electrode11, as shown in FIG. 7(b), thereby resulting in display non-uniformity.Note that if a disclination were to occur substantially in the sameregion in each pixel, one might consider such disclinations not to beobserved as significant non-uniformity. Even then, the presence ofdisclination leads to a decrease in brightness. Therefore, it ispreferred to suppress the occurrence of a disclination itself.

Now, referring to FIG. 8 and FIG. 9, the results of an orientationsimulation for verifying the orientation of liquid crystal molecules inthe presence of an applied voltage will be described, for each of aliquid crystal display device 1000 of a common 4D-RTN mode and theliquid crystal display device 900 of the reference example. FIGS. 8(a)to (d) are views relating to the liquid crystal display device 1000 ofthe 4D-RTN mode, and FIGS. 9(a) to (d) are views relating to the liquidcrystal display device 900 of the reference example. FIG. 8(a) and FIG.9(a) are calculation mask diagrams, showing the pre-tilt directions PD1and PD2 defined by the photo-alignment film on the active matrixsubstrate side, together with the pre-tilt directions PD3 and PD4defined by the photo-alignment film on the counter substrate side. FIG.8(b) and FIG. 9(b) show transmittance simulation results obtained whenlinear polarizers arranged so that the transmission axes P1 and P2 aregenerally orthogonal to each other are used as polarizers. FIG. 8(c) andFIG. 9(c) show transmittance simulation results obtained when circularpolarizers are used as polarizers. FIG. 8(d) and FIG. 9(d) each show theorientation distribution within a pixel, showing, by using an arrow, thegeneral orientation direction of liquid crystal molecules (which can besaid to be the azimuth direction of the director) in each liquid crystaldomain. Note that the calculation conditions for the orientationsimulation are as shown in Table 1 below.

TABLE 1 Calculation condition Dielectric constant of Dielectric constantin molecule 3.7 liquid crystal material major axis direction ε//Dielectric constant in molecule 7.8 minor axis direction ε⊥ Refractiveindex of Refractive index in molecule 1.6061 liquid crystal materialmajor axis direction n// Refractive index in molecule 1.4862 minor axisdirection n⊥ Cell thickness (thickness of 3.1 μm liquid crystal layer)Voltage applied to liquid 3.75 V crystal layer Pixel pitch 28.25 μm ×84.75 μm (corresponding to screen resolution of 300 ppi)

As can be seen from FIG. 8(a) and FIG. 9(a), the shape of the pixelelectrode, etc., is the substantially same between the liquid crystaldisplay device pixel 1000 of the 4D-RTN mode and the liquid crystaldisplay device 900 of the reference example.

Where circular polarizers are used, although a small dark spot ispresent at the center of the pixel, a high brightness is realized inother areas of the pixel, for the liquid crystal display device pixel1000 of the 4D-RTN mode and for the liquid crystal display device 900 ofthe reference example, as shown in FIG. 8(c) and FIG. 9(c).

Where linear polarizers are used, there appear swastika-shaped darklines that is characteristic of the 4D-RTN mode, as shown in FIG. 8(b),with the liquid crystal display device pixel 1000 of the 4D-RTN mode. Onthe other hand, with the liquid crystal display device 900 of thereference example, there appear dark lines of substantially the samepattern, as shown in FIG. 9(b), though the dark lines are locatedslightly different from the liquid crystal display device pixel 1000 ofthe 4D-RTN mode. Therefore, it is believed that a 4D structure is formedalso with the liquid crystal display device 900 of the referenceexample.

As can be seen from FIG. 8(d) and FIG. 9(d), it was confirmed thatliquid crystal molecules were oriented generally in four directions forthe liquid crystal display device pixel 1000 of the 4D-RTN mode and forthe liquid crystal display device 900 of the reference example.

Then, referring now to FIG. 10, the reason why four liquid crystaldomains are formed in a pixel 901 of the liquid crystal display device900 of the reference example will be described.

As shown in FIG. 10, in the liquid crystal display device 900 of thereference example, the outer perimeter of the pixel electrode 11includes a first edge portion 11 e 1 and a second edge portion 11 e 2.The first edge portion 11 e 1 lies near the liquid crystal domain A, anda second edge portion 11 e 2 lies near the liquid crystal domain C.

The direction e1, which is orthogonal to a first edge portion 11 e 1 andwhich extends toward the inside of the pixel electrode 11, is oppositeto the first pre-tilt direction PD1. The direction e2, which isorthogonal to the second edge portion 11 e 2 and which extends towardthe inside of the pixel electrode 11, is opposite to the second pre-tiltdirection PD2.

With the outer perimeter of the pixel electrode 11 including the firstedge portion 11 e 1 and the second edge portion 11 e 2, there are formedfour liquid crystal domains in the presence of an applied voltage. Thereason for this will now be described in greater detail with referenceto FIG. 11 and FIG. 12.

FIG. 11(a) is a plan view showing one pixel 901, and FIG. 11(a) shows,by solid-line arrows A1 and A2, the alignment regulating forces from thefirst photo-alignment film 12 and the second photo-alignment film 22.FIG. 11(b) is a cross-sectional view taken along line 11B-11B′ of FIG.11(a), showing the orientation of the liquid crystal molecules 31 in theabsence of an applied voltage. As can be seen from FIG. 11(b), theliquid crystal molecules 31 are pre-tilted at a predetermined angle(pre-tilt angle) θ in a predetermined direction by virtue of thealignment regulating forces A1 and A2 from the first photo-alignmentfilm 12 and the second photo-alignment film 22 in the absence of anapplied voltage. For example, in the upper half of the pixel 901, theliquid crystal molecules 31 are tilted leftward with respect to thesubstrate surface normal direction.

FIG. 12(a) is a plan view showing one pixel 901, and FIG. 12(a) shows,by broken-line arrows B1, B2, B3 and B4, the alignment regulating forcesfrom an oblique electric field produced in the vicinity of the outerperimeter of the pixel electrode 11, as well as the alignment regulatingforces A1 and A2 from the first photo-alignment film 12 and the secondphoto-alignment film 22. FIG. 12(b) is a cross-sectional view takenalong line 12B-12B′ of FIG. 12(a), showing the orientation of the liquidcrystal molecules 31 in the presence of an applied voltage (withouttaking into consideration the alignment regulating forces A1 and A2 fromthe first photo-alignment film 12 and the second photo-alignment film22). As can be seen from FIG. 12(b), the liquid crystal molecules 31,which have a negative dielectric anisotropy, are oriented so as to beperpendicular to the electric force lines E in the presence of anapplied voltage. Therefore, in the vicinity of the outer perimeter ofthe pixel electrode 11, there are alignment regulating forces urging theliquid crystal molecules 31 to tilt toward the inside of the pixelelectrode 11 (the alignment regulating forces B1, B2, B3 and B4 shown inFIG. 12(a)).

Therefore, in portions of the pixel 901 (herein, an upper left portionand a lower right portion of the pixel 901, i.e., regions R1 and R2shown in FIG. 12(a)), the directions of the alignment regulating forcesfrom the first photo-alignment film 12 and the second photo-alignmentfilm (directions that coincide with the first pre-tilt direction PD1 andthe second pre-tilt direction PD2) are opposite to the directions of thealignment regulating forces from an oblique electric field (directionswhich are orthogonal to the edge portion of the pixel electrode 11 andwhich extend toward the inside of the pixel electrode 11).

FIG. 13(a) shows, on an enlarged scale, an upper left portion of thepixel 901. In this portion, the alignment regulating force A1 from thefirst photo-alignment film 12 and the second photo-alignment film 22 andthe alignment regulating forces B1 and B4 from an oblique electric fieldinteract with each other, thereby tilting the liquid crystal molecules31 downward (the direction C1).

FIG. 13(b) shows, on an enlarged scale, a lower right portion of thepixel 901. In this portion, the alignment regulating force A2 from thefirst photo-alignment film 12 and the second photo-alignment film 22 andthe alignment regulating forces B2 and B3 from an oblique electric fieldinteract with each other, thereby tilting the liquid crystal molecules31 upward (the direction C2).

With the mechanism described above, in the liquid crystal domains A andC in the vicinity of the first edge portion 11 e 1 and the second edgeportion 11 e 2, the liquid crystal molecules 31 are oriented in thedisplay plane vertical directions (the generally 270° direction and thegenerally 90° direction) in the presence of an applied voltage.Therefore, four liquid crystal domains A, B, C and D are formed withinthe pixel 1, of which the azimuth directions of the directors aredifferent from each other.

Thus, with the liquid crystal display device 900 of the referenceexample, there are actually formed four liquid crystal domains A, B, Cand D within each pixel 901 in the presence of an applied voltage,thereby realizing a sufficiently high viewing angle characteristic.However, with the liquid crystal display device 900 of the referenceexample, a disclination may occur in particular regions (the regions R1and R2 in FIG. 12(a)) near the outer perimeter of the pixel electrode11. A disclination is thought to occur for the following reason.

In an upper left portion of the pixel 901 (near the first edge portion11 e 1), the alignment regulating force A1 from the firstphoto-alignment film 12 and the second photo-alignment film 22 and twodifferent (rightward and downward) alignment regulating forces B1 and B4from an oblique electric field act upon the liquid crystal molecules 31,as already described above. It is believed that since the alignmentregulating force A1 from the first photo-alignment film 12 and thesecond photo-alignment film 22 and one (the rightward alignmentregulating force) B1 of the two different alignment regulating forces B1and B4 from the oblique electric field are in opposite directions fromeach other, the remaining alignment regulating force (the downwardalignment regulating force from the oblique electric field) B4 becomesdominant, thereby tilting the liquid crystal molecules 31 downward.However, it is believed that where the dominant alignment regulatingforce B4 is small (not sufficiently large), it is difficult to uniformlytilt downward the liquid crystal molecules 31 across the entire regionR1. Therefore, as shown in FIG. 14, the liquid crystal molecules 31 maybe oriented randomly downward or upward in the region R1 near the firstedge portion 11 e 1, thereby causing a disclination.

In a lower right portion of the pixel 901 (near the second edge portion11 e 2), the alignment regulating force A2 from the firstphoto-alignment film 12 and the second photo-alignment film 22 and thetwo different (upward and leftward) alignment regulating forces B2 andB3 from the oblique electric field act upon the liquid crystal molecules31, as already described above. It is believed that since the alignmentregulating force A2 from the first photo-alignment film 12 and thesecond photo-alignment film 22 and one (the leftward alignmentregulating force) B3 of the two different alignment regulating forces B2and B3 from the oblique electric field are in opposite directions fromeach other, the remaining alignment regulating force (the upwardalignment regulating force from the oblique electric field) B2 becomesdominant, thereby tilting the liquid crystal molecules 31 upward.However, it is believed that where the dominant alignment regulatingforce B2 is small (not sufficiently large), it is difficult to uniformlytilt upward the liquid crystal molecules 31 across the entire region R2.Therefore, as shown in FIG. 14, the liquid crystal molecules 31 may beoriented randomly upward or downward in the region R2 near the secondedge portion 11 e 2, thereby causing a disclination.

As described above, with the liquid crystal display device 900 of thereference example, a disclination may occur in regions R1 and R2 nearthe first edge portion 11 e 1 and the second edge portion 11 e 2 of thepixel electrode 11. In contrast, with the provision of the cut-offportions (the first cut-off portion 11 a 1 and the second cut-offportion 11 a 2 shown in FIG. 4, etc.) in particular regions of the pixelelectrode 11 in the liquid crystal display device 100 of the presentembodiment, the occurrence of a disclination as described above issuppressed. The reason for this will now be described.

As shown in FIG. 15, in the liquid crystal display device 100 of thepresent embodiment, the outer perimeter of the pixel electrode 11includes the first edge portion 11 e 1 and the second edge portion 11 e2. The first edge portion 11 e 1 lies near the liquid crystal domain A,and a second edge portion 11 e 2 lies near the liquid crystal domain C.

The direction e1, which is orthogonal to the first edge portion 11 e 1and which extends toward the inside of the pixel electrode 11, isopposite to the first pre-tilt direction PD1. The direction e2, which isorthogonal to the second edge portion 11 e 2 and which extends towardthe inside of the pixel electrode 11, is opposite to the second pre-tiltdirection PD2.

With the outer perimeter of the pixel electrode 11 including the firstedge portion 11 e 1 and the second edge portion 11 e 2 as describedabove, there are formed four liquid crystal domains A, B, C and D in thepresence of an applied voltage also in the liquid crystal display device100 of the present embodiment, thereby obtaining a sufficiently highviewing angle characteristic, in accordance with a similar mechanism tothat described above for the liquid crystal display device 900 of thereference example.

The pixel electrode 11 of the liquid crystal display device 100 of thepresent embodiment includes the first cut-off portion 11 a 1 provided bycutting off at least a part of the first edge portion 11 e 1, and thesecond cut-off portion 11 a 2 provided by cutting off at least a part ofthe second edge portion 11 e 2. The first cut-off portion 11 a 1 has aright triangle shape obtained by cutting off a corner 11 c 1 of thepixel electrode 11 in the vicinity of the first edge portion 11 e 1. Thesecond cut-off portion 11 a 2 has a right triangle shape obtained bycutting off a corner of the pixel electrode 11 in the vicinity of thesecond edge portion 11 e 2.

Note that as is clear from FIG. 15, the “outer perimeter” of the pixelelectrode 11, as used in the present specification, is defined byportions of the pixel electrode where the conductive layer is present(hereinafter referred to also as “conductive layer portions”) and by thecut-off portions (portions where the conductive layer is absent) 11 a 1and 11 a 2 (i.e., it is not the outer perimeter of only the portionwhere the conductive layer is present). In the configuration illustratedin FIG. 15, the outer perimeter of the pixel electrode 11 is generallyrectangular.

Now, referring to FIG. 16, the reason why the occurrence of adisclination is suppressed by the provision of the first cut-off portion11 a 1 and the second cut-off portion 11 a 2 as described above will bedescribed.

With the provision of the first cut-off portion 11 a 1, the edge of theconductive layer portion of the pixel electrode 11 is tilted, near thefirst edge portion 11 e 1, with respect to the display plane up-downdirection (vertical direction). Therefore, as shown in FIG. 16, thealignment regulating force B1 from the oblique electric field near thefirst edge portion 11 e 1 is in a direction tilted with respect to thedisplay plane up-down direction (vertical direction).

The alignment regulating force B1 can be decomposed into the horizontaldirection component B1 _(H) and the vertical direction component B1_(V), as shown in FIG. 17(a). The horizontal direction component B1 _(H)of the alignment regulating force B1 and the alignment regulating forceA1 from the first photo-alignment film 12 and the second photo-alignmentfilm 22 are in opposite directions from each other, and thus act tooffset each other. In contrast, the vertical direction component B1 _(V)of the alignment regulating force B1 is in the same direction as thealignment regulating force B4, which is the dominant alignmentregulating force in the region R1, and thus serves to reinforce thealignment regulating force tilting the liquid crystal molecules 31downward. Therefore, it is possible to uniformly tilt downward theliquid crystal molecules 31 across the entire region R1 near the firstedge portion 11 e 1, thereby suppressing the occurrence of adisclination.

With the provision of the second cut-off portion 11 a 2, the edge of theconductive layer portion of the pixel electrode 11 near the second edgeportion 11 e 2 is tilted with respect to the display plane up-downdirection (vertical direction). Therefore, as shown in FIG. 16, thealignment regulating force B3 from the oblique electric field near thesecond edge portion 11 e 2 is in a direction tilted with respect to thedisplay plane up-down direction (vertical direction).

The alignment regulating force B3 can be decomposed into the horizontaldirection component B3 _(H) and the vertical direction component B3_(V), as shown in FIG. 17(b). The horizontal direction component B3 _(H)of the alignment regulating force B3 and the alignment regulating forceA2 from the first photo-alignment film 12 and the second photo-alignmentfilm 22 are in opposite directions from each other, and thus act tooffset each other. In contrast, the vertical direction component B3 _(V)of the alignment regulating force B3 is in the same direction as thealignment regulating force B2, which is the dominant alignmentregulating force in the region R2, and thus serves to reinforce thealignment regulating force tilting the liquid crystal molecules 31upward. Therefore, it is possible to uniformly tilt upward the liquidcrystal molecules 31 across the entire region R2 near the second edgeportion 11 e 2, thereby suppressing the occurrence of a disclination.

Next, a preferred configuration of the pixel electrode 11 having thefirst cut-off portion 11 a 1 and the second cut-off portion 11 a 2 willbe described.

Where a₁, a₂ and b denote the lengths of the first cut-off portion 11 a1, the second cut-off portion 11 a 2 and the pixel electrode 11,respectively, along the direction orthogonal to the first pre-tiltdirection PD1 and the second pre-tilt direction PD2 (the directionparallel to the first edge portion 11 e 1 and the second edge portion 11e 2), as shown in FIG. 18, the length a₁ of the first cut-off portion 11a 1, the length a₂ of the second cut-off portion 11 a 2 and the length bof the pixel electrode 11 preferably satisfy relationships ofExpressions (1) and (2) below. That is, it is preferred that the lengtha₁ of the first cut-off portion 11 a 1 and the length a₂ of the secondcut-off portion 11 a 2 are each 25% or more (¼ or more) of the length bof the pixel electrode 11.

a ₁ /b≧0.25  (1)

a ₂ /b≧0.25  (2)

Satisfying Expressions (1) and (2), it is possible to more reliablysuppress the occurrence of a disclination. Results verifying this willlater be discussed in detail. Note that the left-hand side ofExpressions (1) and (2) (“a₁/b”, “a₂/b”) may be referred to hereinbelowas the “cut-off ratio”.

As can be seen from the description above, the first cut-off portion 11a 1 and the second cut-off portion 11 a 2 can be formed by cutting offedge portions (the first edge portion 11 e 1 and the second edge portion11 e 2) near regions where the alignment regulating force from the firstphoto-alignment film 12 and the second photo-alignment film and thealignment regulating force from the oblique electric field are inopposite directions from each other, and it is not necessary to cut offedge portions (edge portions 11 e 3 and 11 e 4 in FIG. 16) near regionswhere the alignment regulating force from the first photo-alignment film12 and the second photo-alignment film 22 and the alignment regulatingforce from the oblique electric field are in the same direction. Cuttingoff these edge portions 11 e 3 and 11 e 4 will not pose a problem inview of the orientation of the liquid crystal molecules 31 (nodisclination will newly occur). Nevertheless, an increase in theproportion of the area cut off along the outer perimeter of the pixelelectrode 11 may possibly increase the area where the electric field isnot directly applied to the liquid crystal layer 30, thereby loweringthe display brightness. Therefore, in order to realize bright display,it is preferred that the length a₁ of the first cut-off portion 11 a 1,the length a₂ of the second cut-off portion 11 a 2 and the length b ofthe pixel electrode 11 satisfy relationships of Expressions (3) and (4)below. That is, it is preferred that the cut-off ratios (a₁/b and a₂/b)of the first cut-off portion 11 a 1 and the second cut-off portion 11 a2 are each 0.5 or less. In other words, it is preferred that the lengtha₁ of the first cut-off portion 11 a 1 and the length a₂ of the secondcut-off portion 11 a 2 are each 50% or less (½ or less) of the length bof the pixel electrode 11.

a ₁ /b≦0.5  (3)

a ₂ /b≦0.5  (4)

Where φ₁ denotes the angle formed between the hypotenuse of the firstcut-off portion 11 a 1 and the first edge portion 11 e 1 and φ₂ denotesthe angle formed between the hypotenuse of the second cut-off portion 11a 2 and the second edge portion 11 e 2, as shown in FIG. 18, it ispreferred that these angles (hereinafter referred to also as “cut-offangles”) φ₁ and φ₂ satisfy relationships of Expressions (5) and (6)below.

φ₁≧1°  (5)

φ₂≧1°  (6)

Note that even if the cut-off angles φ₁ and φ₂ are less than 1° (φ₁<1°,φ₂<1°), it is believed that in theory there is realized an effect ofsuppressing the occurrence of a disclination. Note however that it maybe difficult to actually form the first cut-off portion 11 a 1 and thesecond cut-off portion 11 a 2 such that the cut-off angles φ₁ and φ₂ areless than 1° by patterning a conductive film (the cut-off angles φ₁ andφ₂ may become substantially 0°).

Now, results obtained by verifying the presence/absence of adisclination (orientation defect) through an orientation simulationwhile varying the cut-off ratios and the cut-off angles of the firstcut-off portion 11 a 1 and the second cut-off portion 11 a 2 will bediscussed. Table 2 below and FIG. 19 to FIG. 32 show the simulationresults. In Table 2, the symbol “x” indicates that a significantdisclination occurred, the symbol “Δ” indicates that a disclinationoccurred albeit insignificantly, and the symbol “∘” indicates that nodisclination occurred. In Table 2, the symbol “” indicates that it wasclearly unlikely that a disclination would occur even thoughcalculations were not done (with the amount of cut-off of the edgeportion being even larger than those samples labeled “∘”). In FIG. 19 toFIG. 32, (a) is a calculation mask diagram. In FIG. 19 to FIG. 32, (b)is a view illustrating transmittance simulation results when usinglinear polarizers arranged so that transmission axes thereof aregenerally orthogonal to each other. In FIG. 19 to FIG. 32, (c) is a viewshowing the orientation distribution within a pixel. Note that thecalculation conditions for the orientation simulation are as shown inTable 3 below.

TABLE 2 Cut-off ratio (a₁/b, a₂/b) 0 0.05 0.10 0.25 0.50 Cut-off 0° Δ —— — — angle 1° Δ — Δ ∘ ∘ (φ₁, φ₂) 2° Δ — Δ ∘ ∘ 5° — — Δ ∘  15° — Δ Δ  30° — Δ Δ   45° — Δ Δ  

TABLE 3 Calculation condition Dielectric constant of Dielectric constantin molecule 3.7 liquid crystal material major axis direction ε//Dielectric constant in molecule 7.8 minor axis direction ε⊥ Refractiveindex of Refractive index in molecule 1.6061 liquid crystal materialmajor axis direction n// Refractive index in molecule 1.4862 minor axisdirection n⊥ Cell thickness (thickness of 3.1 μm liquid crystal layer)Voltage applied to liquid 0 V → 3.75 V crystal layer Varied slowly tovoltage value where brightness corresponding to white display isobtained Pixel pitch 88.5 μm × 265.5 μm Pre-tile angle 2.0° Intervalbetween adjacent   6 μm pixel electrodes

As can be seen from Table 2 and FIG. 19, a significant disclinationoccurred when the first cut-off portion 11 a 1 and the second cut-offportion 11 a 2 were not formed in the pixel electrode 11 (where thecut-off ratio was 0). As can be seen from Table 2, FIG. 22, FIG. 25 andFIG. 27 to FIG. 32, a disclination occurred albeit insignificantly whenthe cut-off ratio of the first cut-off portion 11 a 1 and the secondcut-off portion 11 a 2 was 0.10 or less (although it exceeded 0). Incontrast, as can be seen from Table 2, FIG. 20, FIG. 21, FIG. 23, FIG.24 and FIG. 26, no disclination occurred when the cut-off ratio of thefirst cut-off portion 11 a 1 and the second cut-off portion 11 a 2 was0.25 or more.

Thus, with the cut-off ratio of the first cut-off portion 11 a 1 and thesecond cut-off portion 11 a 2 being 0.25 or more, it is possible to morereliably suppress the occurrence of a disclination. The reason for thiswill now be described, referring to FIG. 33.

FIG. 33 shows a case where the cut-off ratio (a₁/b, a₂/b) is 0.25. Onecan assume that in this case, near the first edge portion 11 e 1, thereis a region R1 a where the orientation direction of the liquid crystalmolecules 31 is not uniformly defined and a region R1 b where theorientation direction of the liquid crystal molecules 31 is uniformlydefined, where the region R1 a and the region R1 b are of the same size.Then, it is believed that the former region R1 a is influenced by thelatter region R1 b so that the orientation direction of the liquidcrystal molecules 31 is uniformly defined also in the former region R1a, and as a result, the orientation direction of the liquid crystalmolecules 31 is uniformly defined near the entire first edge portion 11e 1. Note that one may possibly assume that it is the latter region R1 bthat is influenced by the former region R1 a, but it is believed thatthe orientation direction is uniformly defined when the cut-off ratio is0.25 because the liquid crystal layer 30 is basically more stableenergy-wise with no disclination.

Similarly, one can assume that near the second edge portion 11 e 2,there is a region R2 a where the orientation direction of the liquidcrystal molecules 31 is not uniformly defined and a region R2 b wherethe orientation direction of the liquid crystal molecules 31 isuniformly defined, where the region R2 a and the region R2 b are of thesame size. Then, it is believed that the former region R2 a isinfluenced by the latter region R2 b so that the orientation directionof the liquid crystal molecules 31 is uniformly defined also in theformer region R2 a, and as a result, the orientation direction of theliquid crystal molecules 31 is uniformly defined near the entire secondedge portion 11 e 2.

It is believed that if the cut-off ratio is less than 0.25, in contrast,the influence of the regions R1 a and R2 a on the orientation of theliquid crystal molecules 31 may be greater than that of the regions R1 band R2 b, and the orientation direction is less likely uniformlydefined. The results shown in Table 2 are well in line with thisassumption.

Note that as can be seen from the description above, the cut-off ratioof the first cut-off portion 11 a 1 being 0.25 or more (a₁/b≧0.25) meansthat the length a₁ of the first cut-off portion 11 a 1 is greater thanor equal to one half of the length of the first edge portion 11 e 1.Similarly, the cut-off ratio of the second cut-off portion 11 a 2 being0.25 or more (a₂/b≧0.25) means that the length a₂ of the second cut-offportion 11 a 2 is greater than or equal to one half of the length of thesecond edge portion 11 e 2.

As already described above, with the liquid crystal display device 100of the present embodiment, as with the liquid crystal display device 900of the reference example, there are formed four liquid crystal domainsA, B, C and D, thereby realizing a high viewing angle characteristic.Now, the results of an orientation simulation and an optical simulationperformed with various screen resolutions (pixel pitches) for verifyingthe viewing angle characteristic of the liquid crystal display device900 of the reference example will be described.

The simulation was performed for six screen resolutions as ReferenceExamples 1 to 6. The simulation was also performed for the liquidcrystal display device 1000 of a common 4D-RTN mode, as ComparativeExample 1. The screen resolutions and the pixel pitches (pixel sizes) ofReference Examples 1 to 6 and Comparative Example 1 are as shown inTable 4 below. The simulation was performed, where the liquid crystalmaterial was a nematic liquid crystal material having a refractive indexanisotropy Δn=0.1199, and the thickness of the liquid crystal layer (thecell thickness) was 3.1 μm. Moreover, the interval between adjacentpixel electrodes was 6 μm, and the pre-tilt angle was 2.4°. A regionwhere the photo-alignment film is exposed redundantly (a redundantexposure region) was formed with a width of 20 μm in the central portionof each pixel, and the pre-tilt angle at the center of the redundantexposure region was set to 0°. The white voltage (highest gray levelvoltage) was set to 3.9 V based on the measured values for an actualprototype 4.18-inch panel.

TABLE 4 Screen resolution Pixel pitch Reference 1 500 ppi 16.93 μm ×50.8 μm  Example 2 400 ppi 21.16 μm × 63.5 μm  3 300 ppi 28.25 μm ×84.75 μm  4 217 ppi 39 μm × 117 μm 5 160 ppi 52.91 μm × 158.75 μm 6  96ppi 88.5 μm × 265.5 μm Comparative  96 ppi 88.5 μm × 265.5 μm Example 1(4D-RTN)

Referring to FIG. 34 to FIG. 40, simulation results for ReferenceExamples 1 to 6 and Comparative Example 1 will be described. FIG. 34corresponds to Reference Example 1, FIG. 35 corresponds to ReferenceExample 2, and FIG. 36 corresponds to Reference Example 3. FIG. 37corresponds to Reference Example 4, FIG. 38 corresponds to ReferenceExample 5, FIG. 39 corresponds to Reference Example 6, and FIG. 40corresponds to Comparative Example 1.

In FIG. 34 to FIG. 40, (a) shows a calculation mask diagram. In FIG. 34to FIG. 40, (b) shows transmittance simulation results obtained whencircular polarizers are used as polarizers. In FIG. 34 to FIG. 40, (c)shows transmittance simulation results obtained when linear polarizersarranged so that the transmission axes are generally orthogonal to eachother are used as polarizers. In FIG. 34 to FIG. 40, (d) shows theorientation distribution within a pixel, showing, by using an arrow, thegeneral orientation direction of liquid crystal molecules (which can besaid to be the azimuth direction of the director) in each liquid crystaldomain. In FIG. 34 to FIG. 40, (e) is a graph showing the relationshipbetween the gray level and the brightness (normalized with 1 being thebrightness of the white display) as observed from the front direction,for a diagonally right 60° direction (the direction obtained by tiltingthe viewing angle by 60° rightward), and for a diagonally upper 60°direction (the direction obtained by tilting the viewing angle by 60°upward), indicating how much the γ characteristic (the gray leveldependence of the brightness) shifts when observed from a diagonaldirection than when observed from the front direction. Referparticularly to (e) of FIG. 34 to FIG. 40 in conjunction with thefollowing description.

As can be seen from FIG. 40, in Comparative Example 1 (common 4D-RTNmode), even though the screen resolution is relatively low (96 ppi, witha pixel pitch of 88.5 μm×265.5 μm), there is substantially no brightnessdifference for substantially every gray level between when observed fromthe diagonally right 60° direction and when observed from the diagonallyupper 60° direction. This indicates that a desirable viewing anglecharacteristic is obtained, i.e., there is a small azimuth angledependence of the γ characteristic shift (γ shift) when observed from adiagonal direction.

In contrast, as can be seen from FIG. 39, in Reference Example 6 wherethe screen resolution is the same as Comparative Example 1, thebrightness difference between when observed from the diagonally right60° direction and when observed from the diagonally upper 60° directionis greater than that for Comparative Example 1.

As can be seen from FIG. 34 to FIG. 39, in Reference Examples 1 to 6,the brightness difference between when observed from the diagonallyright 60° direction and when observed from the diagonally upper 60°direction tends to decrease as the screen resolution increases (as thepixel pitch decreases). It can be seen that when the screen definitionis 200 ppi or more (Reference Examples 1 to 4), the azimuth angledependence of the γ shift is sufficiently small, thereby realizing asufficiently high viewing angle characteristic.

Note that even though Reference Example 6 has a lower screen resolutionthan Reference Example 5, the brightness difference between whenobserved from the diagonally right 60° direction and when observed fromthe diagonally upper 60° direction is smaller than Reference Example 5.It is believed that this is because the calculation is done in ReferenceExample 6 using such a pattern that the inside of the pixel is partiallyshaded.

Then, referring now to FIGS. 41(a), (b) and (c), the reason why a higherviewing angle characteristic is realized as the screen resolutionincreases. FIGS. 41(a), (b) and (c) are enlarged versions of FIG. 40(d),FIG. 39(d) and FIG. 35(d), showing the orientation distribution within apixel for Comparative Example 1, Reference Example 6 and ReferenceExample 2, respectively.

In Comparative Example 1, as can be seen from FIG. 41(a), four regionsin which the liquid crystal molecules are oriented in the lower leftdirection (a generally 225° direction), the lower right direction (agenerally 315° direction), the upper right direction (a generally 45°direction) and the upper left direction (a generally 135° direction)account for a majority of the pixel. Although regions in which theliquid crystal molecules are oriented in the downward direction (agenerally 270° direction) and the upward direction (a generally 90°direction) are present in an upper left portion and a lower rightportion of the pixel, these regions account for a small proportion ofthe pixel. Therefore, it can be regarded that the liquid crystalmolecules are oriented generally in four directions within a pixel, andthe four regions in which the liquid crystal molecules are oriented inthe lower left direction, the lower right direction, the upper rightdirection and the upper left direction are dominant in terms of theoptical characteristics.

In Reference Example 6, in contrast, as can be seen from FIG. 41(b), tworegions in which the liquid crystal molecules are oriented in the leftdirection (a generally 180° direction) and the right direction (agenerally 0° direction) account for a majority of the pixel. Althoughregions in which the liquid crystal molecules are oriented in thedownward direction (a generally 270° direction) and the upward direction(a generally 90° direction) are present in an upper left portion and alower right portion of the pixel, these regions account for a smallproportion of the pixel. Therefore, it can be regarded that the liquidcrystal molecules are oriented generally in two directions within apixel, and the two regions in which the liquid crystal molecules areoriented in the left direction and the right direction are dominant interms of the optical characteristics.

In Reference Example 2, in contrast, as can be seen from FIG. 41(c),regions in which the liquid crystal molecules are oriented in thedownward direction and the upward direction account for a higherproportion of the pixel than in Reference Example 6. Therefore, it canbe regarded that the liquid crystal molecules are oriented generally infour directions within a pixel, and the four regions in which the liquidcrystal molecules are oriented in the downward direction, the rightdirection, the upward direction and the left direction (the four liquidcrystal domains A, B, C and D) are dominant in terms of the opticalcharacteristics.

As described above, as the screen resolution is higher (i.e., as thepixel pitch is smaller), the difference in area between the four liquidcrystal domains decreases, improving the viewing angle characteristic.According to a study by the present inventors, it has been found that ifthe screen resolution is 200 ppi or more (if the shorter one of thepixel pitch along the display plane horizontal direction and the pixelpitch along the display plane vertical direction is 42 μm or less), thedifference between the liquid crystal domain of the largest area and theliquid crystal domain of the smallest area can be made relatively small(specifically, 50% or less), and it is possible to realize asufficiently high viewing angle characteristic comparable to that of aliquid crystal display device of a common 4D-RTN mode. Note thatalthough verification results for the liquid crystal display device 900of the reference example have been described above, they similarly applyalso to the liquid crystal display device 100 of the present embodiment.That is, also with the liquid crystal display device 100 of the presentembodiment, if the screen resolution is 200 ppi or more (if the shorterone of the pixel pitch along the display plane horizontal direction andthe pixel pitch along the display plane vertical direction is 42 μm orless), it is possible to realize a sufficiently high viewing anglecharacteristic comparable to that of a liquid crystal display device ofa common 4D-RTN mode.

Then, the results of measuring various characteristics of an actualprototype of a liquid crystal display device 900 having a screenresolution of 217 ppi as Reference Example 7 will be described. Theresults of measuring various characteristics of a prototype of a liquidcrystal display device 1000 of a 4D-RTN mode having a screen resolutionof 217 ppi as Comparative Example 2 and a prototype of a liquid crystaldisplay device of a CPA (Continuous Pinwheel Alignment) mode having ascreen resolution of 217 ppi as Comparative Example 3 will also bedescribed. Note that the CPA mode is a type of the VA mode, and isdisclosed in Japanese Laid-Open Patent Publication No. 2003-43525 andJapanese Laid-Open Patent Publication No. 2002-202511, for example.

Table 5 below shows the results of measuring the transmittance and theresponse speed for Reference Example 7, Comparative Example 2 andComparative Example 3. As for the response speed, the rising responsetime Tr when changing the display gray level from 0 gray level to 32gray level at 25° C., and the falling response time Td when changing itfrom 32 gray level to 0 gray level are shown. Table 5 also shows thetemperature of the heating treatment in the re-alignment treatment stepand the pre-tilt angle (average pre-tilt angle) for Reference Example 7and Comparative Example 2. Note that two liquid crystal display devices900 of different heating treatment temperatures were prototyped asReference Example 7.

TABLE 5 Compara- Comparative tive Example 2 Example 3 Reference Example7 (4D-RTN) (CPA) Transmittance 6.6% 6.6% 6.4% Heating treatment 110° C.130° C. 130° C. — (re-alignment treatment) temperature Pre-tilt angle2.0° 1.4-1.5° 0.9° — Response Rising 51.9 msec 59.4 msec 116.6 msec 53.8msec speed response (25° C.) time T_(r) (0→32 gray level) Falling  9.0msec  8.6 msec  10.3 msec 10.2 msec response time T_(d) (32→0 graylevel)

As can be seen from Table 5, the rising response time Tr of ComparativeExample 2 (4D-RTN mode) is longer (twice or more) than ComparativeExample 3 (CPA mode). It is believed that this is because the proportionof the pixel accounted for by the undesirable tilt region increases,thereby reducing the average pre-tilt angle, as already described above.

In contrast, Reference Example 7 realizes substantially the same risingresponse time as that of Comparative Example 3. It is believed that thisis because the lowering of the average pre-tilt angle is suppressed asalready described above.

Reference Example 7 realizes a higher transmittance than that ofComparative Example 3. It is often the case with the CPA mode that analignment regulating means (projections made of a dielectric material oropenings made in the counter electrode) for fixing the center of theaxially symmetric orientation to thereby stabilize the orientation isprovided on the counter substrate side, and this alignment regulatingmeans causes the lowering of the transmittance. In contrast, the liquidcrystal display device 900 of the reference example does not requiresuch an alignment regulating means, and it is therefore possible torealize a high transmittance. As with the liquid crystal display device100 of the present embodiment, it is possible to realize a good responsecharacteristic and a high transmittance.

FIGS. 42(a) and (b) show, for Reference Example 7 and ComparativeExample 2, the relationship between the gray level and the brightness(normalized brightness) when observed from the front direction, thatwhen observed from the diagonally left/right 60° direction, and thatwhen observed from the diagonally upper/lower 60° direction. As can beseen from FIGS. 42(a) and (b), the azimuth angle dependence of the γshift was small and a sufficiently high viewing angle characteristic wasrealized in Reference Example 7, as in Comparative Example 2. Note thatalthough not shown in the figures, a similarly high viewing anglecharacteristic to that of Comparative Example 2 was realized also inComparative Example 3.

In Table 5 above, as can be seen from a comparison between two liquidcrystal display devices 900 prototyped as Reference Example 7, a largeraverage pre-tilt angle can be obtained when the temperature of theheating treatment in the re-alignment treatment step is lower.Specifically, the heating treatment is preferably performed at 110° C.or less. Note however that the effect of the re-alignment treatment maynot be sufficient with a temperature less than T_(NI)+10° C. (whereT_(NI) is the nematic phase-isotropic phase transition temperature ofthe liquid crystal material), and it is therefore preferred that theheating treatment is T_(NI)+10° C. or more.

Note that the reason why a higher average pre-tilt angle can be obtainedwhen the temperature of the heating treatment is lower may be asfollows.

It is believed that a pre-tilt angle is realized (a pre-tilt directionis defined) by a photo-alignment film because when a photo-alignmentfilm (typically made of a polyimide-based material) is irradiated withultraviolet light, the side chain turns toward where the ultravioletlight is coming in by virtue of a photoreaction of the photofunctionalgroup. However, since this reaction is reversible with respect to heat,the pre-tilt angle returns to the original (before the photo-alignmenttreatment) pre-tilt angle (0°, or 90° with respect to the substrateplane) if the heating treatment in the re-alignment treatment step isperformed at a high temperature over a long period of time. Therefore,it is believed that the heating treatment is preferably performed at alower temperature.

Now, referring to FIGS. 43(a) and (b), the basic structure of the CPAmode mentioned in Comparative Example 3 will be described. FIGS. 43(a)and (b) are a cross-sectional view and a plan view, respectively,schematically showing one pixel of a liquid crystal display device 800of the CPA mode.

The liquid crystal display device 800 includes an active matrixsubstrate 810 and a counter substrate 820 arranged so as to oppose eachother, and a liquid crystal layer 830 of a vertical alignment typeprovided therebetween.

The active matrix substrate 810 includes a pixel electrode 811 providedin each pixel, and a vertical alignment film 812 provided between thepixel electrode 811 and the liquid crystal layer 830. The pixelelectrode 811 and the vertical alignment film 812 are supported on atransparent substrate 810 a.

The counter substrate 820 includes a counter electrode 821 opposing thepixel electrode 811, and a vertical alignment film 822 provided betweenthe counter electrode 821 and the liquid crystal layer 830. The counterelectrode 821 and the vertical alignment film 822 are supported on atransparent substrate 820 a. The counter electrode 821 has an opening821 a formed in a region opposing generally the center of the pixelelectrode 811.

When a voltage is applied across the liquid crystal layer 830, liquidcrystal molecules 831 are oriented in an axially symmetric orientation,as shown in FIGS. 43(a) and (b), by the alignment regulating force of anoblique electric field produced in the vicinity of the outer perimeterof the pixel electrode 811 and the alignment regulating force of anoblique electric field produced in the vicinity of the opening 821 a ofthe counter electrode 821.

The opening 821 a of the counter electrode 821 functions to fix thecenter of the axially symmetric orientation and to stabilize theorientation. As an alignment regulating means having such a function, aprojection (referred to also as a rivet) made of a dielectric materialmay also be used instead of the opening 821 a of the counter electrode821. Note however that since the liquid crystal molecules 831 in thevicinity of the alignment regulating means tend not to tilt in thepresence of an applied voltage, thereby lowering the brightness. FIG.43(c) shows the transmittance simulation results when a white voltage isapplied across the liquid crystal layer 830 of the liquid crystaldisplay device 800 (including circular polarizers as polarizers). As canbe seen from FIG. 43(c), a region corresponding to the opening 821 a ofthe counter electrode 821 appears dark, lowering the brightness.

In order to suppress such lowering of the brightness, one may considerreducing the size of the alignment regulating means itself (typicallyabout 10 μm in diameter for the opening 821 a formed in the counterelectrode 821). However, when the size of the alignment regulating meansis reduced, it may become impossible to sufficiently stabilize theorientation due to an insufficient alignment regulating force. Moreover,in order to form a minute alignment regulating means, it is necessary tointroduce a new piece of equipment such as a high-resolution stepper.

Therefore, since the size of the alignment regulating means cannot bereduced below a certain level, if the pixel pitch decreases due to anincrease in the definition, it will increase the proportion of theentire pixel accounted for by the alignment regulating means for fixingthe center. While the CPA mode at present is often employed in medium-to small-size liquid crystal display devices, the brightness will below, with liquid crystal display devices of the CPA mode, if the pixelpitch decreases due to an increase in the definition, for the reasondescribed above.

In contrast, with the liquid crystal display device 900 of the referenceexample, particularly when circular polarizers are used as polarizers,there is little loss of the brightness and it is possible to realize ahigh transmittance, as can be seen from FIG. 8(c), etc. This similarlyapplies to the liquid crystal display device 100 of the presentembodiment.

As already described above, it is believed that the response speed ofthe liquid crystal display device 900 of the reference example and theliquid crystal display device 100 of the present embodiment improvesbecause the area of the undesirable tilt region NGR is smaller than aliquid crystal display device of the 4D-RTN mode. The results of a testby the present inventors checking whether or not undesirable tiltregions NGR are actually formed due to the optical diffractionphenomenon, and the widths of the undesirable pre-tilt regions NGR ifthey are actually formed will be described below.

First as shown in FIG. 44(a), a substrate 710 with a photo-alignmentfilm 712 formed thereon is prepared, and the photo-alignment film 712 ofthe substrate 710 is irradiated with ultraviolet light from a directionindicated by an arrow.

Next, as shown in FIG. 44(b), irradiation with ultraviolet light wasdone from a direction indicated by an arrow (the opposite direction fromthe direction shown in FIG. 44(a)) with some regions of thephoto-alignment film 712 being shaded by a photomask includinglight-blocking portions 715 arranged in a stripe pattern.

Through two exposures as described above, a region (first region) 712 a,which has been irradiated only once with ultraviolet light, and a region(second region) 712 b, which has been irradiated twice with ultravioletlight, are formed on the photo-alignment film 712, as shown in FIG.44(c). The first region 712 a became, through irradiation withultraviolet light in the first exposure step, a region where a largepre-tilt angle (specifically, 2.5°) can be realized. In contrast, thesecond region 712 b was more influenced by irradiation with ultravioletlight in the second exposure step, of the two exposure steps in whichirradiation with ultraviolet light is done from opposite directions, andthe second region 712 b became a region where a small pre-tilt angle(specifically, −0.5°) can be realized.

Two substrates 710 with two different regions 712 a and 712 b formed onthe photo-alignment film 712 were prepared, and they were attachedtogether while being misaligned with each other by a predetermined angle(herein, 2°), as shown in FIG. 45. As for cross sections (taken alongbroken lines shown in the figure) of the panel obtained through theattachment process, there are a cross section (referred to as an“enhanced region”) including regions where the first regions 712 aoppose each other and regions where the second regions 712 b oppose eachother, a cross section (referred to as an “offset region”) onlyincluding regions where the first region 712 a and the second region 712b oppose each other, and a cross section (referred to as an“intermediate region”) where those regions coexist.

FIG. 46 shows the position profile of the pre-tilt angle along thedirection Y (the left-right direction in FIG. 45) for each of theenhanced region, the intermediate region and the offset region. Now, inorder to check the influence of the diffraction of light, the pre-tiltangle in the enhanced region will be discussed. FIG. 46 also showspositions of the light-blocking portions and the light-transmittingportions of the photomask. Note that the pre-tilt angle is 2.55° whenthe entire surface is exposed only once without using a photomask.

As can be seen from FIG. 46, the pre-tilt angle changes from 2.5° to−0.5° in each region which has a width of about 20 to 40 μm and which iscentered about the boundary between the light-blocking portion and thelight-transmitting portion of the photomask. Thus, there actually areregions where the pre-tilt angle lowers due to the optical diffractionphenomenon. Therefore, as with the liquid crystal display device 900 ofthe reference example and the liquid crystal display device 100 of thepresent embodiment, the entire boundary BD1 between the first pre-tiltregion 12 a and the second pre-tilt region 12 b of the firstphoto-alignment film 12 and the entire boundary BD2 between the thirdpre-tilt region 22 a and the fourth pre-tilt region 22 b of the secondphoto-alignment film 22 can be aligned with each other to reduce thearea of the undesirable tilt region NGR, thereby suppressing thelowering of the average pre-tilt angle of the entire pixel.

Then, the results of measuring the distribution of the pre-tilt anglewithin a pixel 901, for two liquid crystal display devices 900prototyped as Reference Example 7 (screen resolution: 217 ppi, pixelpitch: 39 μm×117 μm), will be described. FIGS. 47(a) and (b) are graphsshowing the relationship between the position in the pixel 901 and thepre-tilt angle, for a case where the temperature of the heatingtreatment in the re-alignment treatment step is 110° C. and for a casewhere it is 130° C., respectively (the time is 40 min for both cases).Note that the vertical axis of the graphs of FIGS. 47(a) and (b)represents the tilt angle with respect to the substrate plane. Themeasurement of the pre-tilt angle was done along the direction Y shownin FIG. 48. FIG. 48 is an optical microscope image corresponding to onepixel 901.

It can be seen from FIGS. 47(a) and (b) that a region which has a widthof about 20 μm and which is centered about the boundary between pre-tiltregions has become a region where a sufficient pre-tilt angle cannot berealized (undesirable tilt region), but a sufficiently large pre-tiltangle can be realized in other regions.

It can be seen from a comparison between FIG. 47(a) and FIG. 47(b) thata larger pre-tilt angle can be realized when the temperature of theheating treatment in the re-alignment treatment step is lower.

Note that the description of the present embodiment is directed to acase where the first photo-alignment film 12 and the secondphoto-alignment film 22 are divided into two regions in the up-downdirection in each pixel 1, and the first pre-tilt direction PD1, thesecond pre-tilt direction PD2, the third pre-tilt direction PD3 and thefourth pre-tilt direction PD4 are generally parallel to the left-rightdirection of the display plane, as shown in FIGS. 5(a) and (b). However,the form of alignment division is not limited to those illustratedabove. For example, where each pixel has an horizontally-elongated shape(the length of the pixel along the display plane left-right direction isgreater than the length of the pixel along the display plane up-downdirection), the first photo-alignment film 12 and the secondphoto-alignment film 22 may be divided into two regions in theleft-right direction in each pixel 1 so that the first pre-tiltdirection PD1, the second pre-tilt direction PD2, the third pre-tiltdirection PD3 and the fourth pre-tilt direction PD4 are generallyparallel to the up-down direction of the display plane.

Although the present embodiment is directed to a case where one colordisplay pixel is formed by three pixels and the aspect ratio of onepixel is 3:1, the number of pixels to be included in one color displaypixel and the aspect ratio of one pixel are not limited to thoseillustrated herein.

INDUSTRIAL APPLICABILITY

An embodiment of the present invention is directed to a liquid crystaldisplay device of a VA mode which is suitable for higher-definitionapplications, and in which the pre-tilt direction of liquid crystalmolecules is defined by a photo-alignment film.

REFERENCE SIGNS LIST

-   1 Pixel-   10 Active matrix substrate-   11 Pixel electrode-   11 a 1 First cut-off portion-   11 a ₂ Second cut-off portion-   11 e 1 First edge portion-   11 e 2 Second edge portion-   12 First photo-alignment film-   12 a First pre-tilt region-   12 b Second pre-tilt region-   18, 28 Polarizer (linear polarizer)-   20 Counter substrate-   21 Counter electrode-   22 Second photo-alignment film-   22 a Third pre-tilt region-   22 b Fourth pre-tilt region-   30 Liquid crystal layer-   31 Liquid crystal molecules-   100 Liquid crystal display device-   e1 Direction which is orthogonal to first edge portion and which    extends toward inside of pixel electrode-   e2 Direction which is orthogonal to second edge portion and which    extends toward inside of pixel electrode-   A, B, C, D Liquid crystal domain-   BD1 Boundary between first pre-tilt region and second pre-tilt    region-   BD2 Boundary between third pre-tilt region and fourth pre-tilt    region-   NGR, NGR1, NGR2 Undesirable tilt region-   PD1 First pre-tilt direction-   PD2 Second pre-tilt direction-   PD3 Third pre-tilt direction-   PD4 Fourth pre-tilt direction-   P1, P2 Transmission axis of linear polarizer

1. A liquid crystal display device including a plurality of pixelsarranged in a matrix pattern, the liquid crystal display devicecomprising: a first substrate and a second substrate arranged so as tooppose each other; and a liquid crystal layer of a vertical alignmenttype provided between the first substrate and the second substrate,wherein: the first substrate includes a pixel electrode provided in eachof the plurality of pixels, and a first photo-alignment film providedbetween the pixel electrode and the liquid crystal layer; the secondsubstrate includes a counter electrode opposing the pixel electrode, anda second photo-alignment film provided between the counter electrode andthe liquid crystal layer; the first photo-alignment film has, in each ofthe plurality of pixels, a first pre-tilt region defining a firstpre-tilt direction, and a second pre-tilt region defining a secondpre-tilt direction, which is anti-parallel to the first pre-tiltdirection; the second photo-alignment film has, in each of the pluralityof pixels, a third pre-tilt region defining a third pre-tilt direction,and a fourth pre-tilt region defining a fourth pre-tilt direction, whichis anti-parallel to the third pre-tilt direction; as seen from a displayplane normal direction, an entire boundary between the first pre-tiltregion and the second pre-tilt region of the first photo-alignment filmand an entire boundary between the third pre-tilt region and the fourthpre-tilt region of the second photo-alignment film are aligned with eachother; an outer perimeter of the pixel electrode includes a first edgeportion and a second edge portion; a direction which is orthogonal tothe first edge portion and which extends toward inside of the pixelelectrode is opposite to the first pre-tilt direction; a direction whichis orthogonal to the second edge portion and which extends toward insidethe pixel electrode is opposite to the second pre-tilt direction; andthe pixel electrode includes a first cut-off portion provided by cuttingoff at least a part of the first edge portion, and a second cut-offportion provided by cutting off at least a part of the second edgeportion.
 2. The liquid crystal display device according to claim 1,wherein: the first cut-off portion has a right triangle shape obtainedby cutting off a corner of the pixel electrode in the vicinity of thefirst edge portion; and the second cut-off portion has a right triangleshape obtained by cutting off a corner of the pixel electrode in thevicinity of the second edge portion.
 3. The liquid crystal displaydevice according to claim 2, wherein: where a₁, a₂ and b denote lengthsof the first cut-off portion, the second cut-off portion and the pixelelectrode, respectively, along a direction orthogonal to the firstpre-tilt direction and the second pre-tilt direction, the length a₁ ofthe first cut-off portion, the length a₂ of the second cut-off portionand the length b of the pixel electrode satisfy relationships a₁/b≧0.25and a₂/b≧0.25.
 4. The liquid crystal display device according to claim3, wherein the length a₁ of the first cut-off portion, the length a₂ ofthe second cut-off portion and the length b of the pixel electrodesatisfy relationships a₁/b≦0.5 and a₂/b≦0.5.
 5. The liquid crystaldisplay device according to claim 2, wherein an angle φ₁ formed betweena hypotenuse of the first cut-off portion and the first edge portion andan angle φ₂ formed between a hypotenuse of the second cut-off portionand the second edge portion satisfy relationships φ₁≧1° and φ₂≧1°. 6.The liquid crystal display device according to claim 1, wherein: as seenfrom the display plane normal direction, the first pre-tilt region ofthe first photo-alignment film and the third pre-tilt region of thesecond photo-alignment film are aligned with each other and the secondpre-tilt region of the first photo-alignment film and the fourthpre-tilt region of the second photo-alignment film are aligned with eachother; and the third pre-tilt direction is anti-parallel to the firstpre-tilt direction, and the fourth pre-tilt direction is anti-parallelto the second pre-tilt direction.
 7. The liquid crystal display deviceaccording to claim 1, wherein: when a voltage is applied between thepixel electrode and the counter electrode, four liquid crystal domainsare formed in the liquid crystal layer in each of the plurality ofpixels; and azimuth directions of four directors representingorientation directions of liquid crystal molecules included in the fourliquid crystal domains, respectively, are different from each other. 8.The liquid crystal display device according to claim 1, furthercomprising: a pair of linear polarizers which are arranged so as tooppose each other with the liquid crystal layer interposed therebetweenand so that transmission axes thereof are generally orthogonal to eachother, wherein the transmission axes of the pair of linear polarizersform an angle of generally 45° with respect to the first pre-tiltdirection.
 9. The liquid crystal display device according to claim 1,further comprising a pair of circular polarizers opposing each otherwith the liquid crystal layer interposed therebetween.
 10. The liquidcrystal display device according to claim 1, wherein the liquid crystallayer includes liquid crystal molecules having a negative dielectricanisotropy.
 11. The liquid crystal display device according to claim 1,wherein a shorter one of a pixel pitch along a display plane horizontaldirection and a pixel pitch along a display plane vertical direction is42 μm or less.
 12. The liquid crystal display device according to claim1, wherein a screen resolution is 200 ppi or more.